Acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS), catalyzes the biosynthesis of the branched chain amino acids valine, leucine and isoleucine (Singh (1999) “Biosynthesis of valine, leucine and isoleucine,” in Plant Amino Acids, Singh, B. K., ed., Marcel Dekker Inc. New York, N.Y., pp. 227-247). A number of compounds such as, for example, sulfonylurea, imidazolinone, triazolopyrimidines, pyrimidinyoxy(thio)benzoates, and/or sulfonylamino-carbonyl-triazonline herbicide, inhibit ALS activity such as for example in plants, enabling the use of such compounds as herbicides. It is desirable to produce crop plants that are tolerant to ALS-inhibiting herbicides, thereby allowing tolerant crop plants to thrive and non-tolerant plants (e.g. weeds) to be killed or severely damaged upon treatment with one or more ALS-inhibiting herbicide.
Tolerance to ALS-inhibiting herbicides has been achieved in at least six crops (Shaner et al. (2007) “Imidazolinone Herbicides,” in Modern Crop Protection Compounds. Volume 1., W. Kramer and U. Schirmer, eds., Wiley-VCH Verlag. Weinheim, Germany, pp. 82-92; Tan et al. (2005) Imidazolinone-tolerant crops: history, current status and future. Pest Manag. Sci. 61:246-257), largely by way of point mutations in ALS genes that render the encoded ALS enzymes insensitive to some or all ALS-inhibiting herbicides. Different mutations in ALS are known to confer tolerance to different ALS herbicides and groups (and/or subgroups) thereof; (see, e.g., Tranel and Wright (2002) Weed Science 50:700-712).
Few if any of the ALS mutations have been fully characterized with respect to the active ingredients to which they confer insensitivity or to the effects they have on the catalytic and cofactor binding properties of the enzyme. For example, some mutations in ALS may lead to altered substrate preference, which may have downstream effects on an organism that expresses the mutated ALS sequence. It is desirable to identify mutations in ALS that not only confer insensitivity to one or more ALS inhibitors but also allow the enzyme to behave kinetically similar to a wild type ALS, thereby minimizing any potentially detrimental effects on an organism expressing the mutated ALS sequence.